RAS proto-oncogene encoded oncoproteins, i.e. the RAS family, represent small guanosine triphosphate (GTP)-binding proteins acting as molecular switches by alternating between an active GTP-bound and an inactive GDP-bound form. RAS proteins are central mediators downstream of growth factor receptor signaling and therefore are critical for controlling cell proliferation, differentiation and apoptosis. RAS can activate several downstream effectors, including the PI3K-Akt-mTOR pathway and the Ras-Raf-MEK-ERK pathway.
Approximately 20% to 30% of all human cancers harbor RAS oncogenic mutations, making RAS variants to highly relevant drivers of cancer. Three different RAS protein isoforms and encoding genes have been identified so far. Of the three RAS isoforms, HRAS (homologous to the oncogene from the Harvey rat sarcoma virus), NRAS (first isolated from a human neuroblastoma), and KRAS (homologous to the oncogene from the Kirsten rat sarcoma virus) including splice variants like K-Ras4A and K-Ras4B, KRAS is the most frequently mutated RAS isoform (86%) and is commonly found in more than 30% of all lung adenocarcinomas. Moreover, hyperactive RAS such as hyperactive KRAS signaling contributes to common immunological and inflammatory disorders, such as rheumatoid arthritis and diabetes. Inhibiting KRAS signaling has been considered for being a mission impossible in the past. Thus, there is a strong need for new effective strategies for inhibiting hyperactive RAS such as hyperactive KRAS signaling.
Mutations of the KRAS gene are usually characterized by a single base missense mutation, which is predominantly found at codons G12, G13 or Q61 residues leading to a constitutive activation of KRAS (Prior, I. A. et al., Cancer research, 2012, 72(10):2457-2467). Constitutive activation of KRAS leads to the persistent stimulation of downstream signaling pathways that promote tumorigenesis (Spiegel, J. et al., Nature chemical biology, 2014, 10(8):613-622, Acquaviva, J., et al. Molecular cancer therapeutics, 2012, 11(12):2633-2643, Shaw, R. J. and Cantley, L. C. Nature, 2006, 441(7092):424-430, Sheridan, C. and Downward, J. The Enzymes, 2013, 34 Pt. B:107-136). Efforts have been made for over three decades to develop effective RAS inhibitors, however no pharmacological inhibitor of the RAS oncoprotein has as yet led to a clinically useful drug (Cox, A. D., Drug discovery, 2014, 13(11):828-851). Numerous strategies, including blocking RAS membrane associations, RAS siRNA technology, targeting RAS downstream effector signaling, inhibiting synthetic lethal interactors with mutant RAS, and suppressing cell metabolism are currently being evaluated in preclinical or clinical studies (Cox, A. D., Drug discovery, 2014, 13(11):828-851, Ostrem, J. M. and Shokat, K. M. Nature reviews. Drugdiscovery. 2016, Lamba, S., et al. Cell reports, 2014, 8(5):1475-1483, Xue, W., et al. Proceedings of the National Academy of Sciences of the United States of America, 2014, 111(34):E3553-3561, Papke, B. Nat Commun, 2016, 7(11360):doi: 10.1038/ncomms11360).
The recent elucidation of the crystalline structure of the cGMP phosphodiesterase (PDEδ) protein and the identification of deltarasin, a molecule that inhibits the binding of PDEδ to activated RAS proteins, has provided hopes in the development of anti-RAS therapy (Chandra, A., et al. Nature cell biology, 2012, 14(2):148-158). Initially, when RAS protein is in the inactive state, it undergoes a rapid series of complex post-translational modifications, which ensures it is capable of binding to the plasma membrane. PDEδ is now regarded as an important chaperone of prenylated small G proteins and a prenyl-binding protein of the RAS superfamily, which can bind to RAS protein and recruit it to the plasma membrane. In particular, PDEδ contains a deep hydrophobic pocket, capable of binding the lipid moiety of farnesyl-acylated proteins such as RAS. Therefore, inhibiting the interaction between RAS-PDEδ could bear a potential therapeutic value.
Deltarasin has been found to bind to the farnesyl-binding pocket of His-tagged PDEδ and to disrupt binding to a biotinylated and farnesylated KRAS peptide (Zimmermann, G., et al., Nature, 2013, 497(7451):638-642). Fluorescence lifetime imaging microscopy (FLIM)-based fluorescence resonance energy transfer (FRET) assays showed that deltarasin inhibits the interaction between KRAS-PDEδ and decreases KRAS binding to the plasma membrane in human ductal adenocarcinoma (PDAC) cell lines harboring KRAS gene mutations, resulting in a reduction of cell proliferation and an induction of apoptosis both in vitro and in vivo. Although the discovery of deltarasin is promising for making the KRAS signaling pathway targetable, the ability of deltarasin to treat other clinical diseases than pancreatic cancer associated with RAS abnormalities, such as suppress lung cancer growth and the factors affecting deltarasin sensitivity have not yet been explored.
Accordingly, there remains a strong need for therapeutic approaches for treating diseases with hyperactive RAS signaling such as hyperactive KRAS signaling, in particular cancer such as non-small cell lung cancer as a leading cause of cancer death and most common type of lung cancer.